Photoshop CC 2017: Slow saving on macOS

Since the update to 2017 CC Photoshop, it is taking far too long to save a moderately sized file. Up to 2 minutes for a 90MB file. I am running a 4 GHz iMac with an SSD Hybrid HD, 32GB of RAM and an AMD Radeon R9 M295X graphics card (4GB). The machine is plenty fast enough. Looking at the activity monitor reveals 30% core use, so not very conclusive. I discarded the settings file on photoshop startup, and was able to save one file at the correct speed, that is to say, about 2 seconds, then it reverted to the slow behaviour. Further discarding of the settings file did not change this behaviour.

OK something has happened and we are back to slow saving in spite of the
downgrade to 2017.0.0. How do I track this problem down? A file that
was saving in 1.5 seconds is now taking 45 seconds to save.

I wonder if your font cache has become corrupted and/or you have a corrupted font. I know that Apple has multiple versions of Helvetica Neue due to some bugs. Try removing that font and then clearing both your font caches (instructions here: https://helpx.adobe.com/photoshop/kb/troubleshoot-fonts-photoshop.html). Then try the validation script. This might point to the problem or could at least rule out one possibility.

I think I found the problem. The photoshop images that were giving me the slow saves were coming from Lightroom. I had a look at the LR export settings for the tiff images and found that they were being compressed as Zip files. Compressing them as LZW or not at all seems to have resolved the slow saving issue.

Although this seems to have resolved the problem, why would resaving a 100MB tiff file with .zip compression take 50-60 seconds as opposed to 3 seconds? just wondering, and not entirely convinced this was the only reason for the slowness.

The save-as-tiff dialog in Photoshop lets you choose LZW vs Zip as compression options and specifically calls out that Zip is smaller files but longer save-time. It's because the zip algorithms used to crush the file size down are much more complex and CPU intensive.

So the extra time is your CPU stuffing a comforter-quilt into a soda can. When that's done, the actual write-to-hard-drive time is probably almost the same.